1997-06-13_REPORT - M1981302 (16)

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' study defines a split flow condition for South Boulder Creek between U.S. Highway 36 and State Highway 93. A split flow condition is caused when physical constraints within the floodplain do not allow floodwaters to remain within the flood channel and the water that normally would remain in the floodplain splits from the floodplain and flows away, sometimes returning to the floodplain and sometimes creating its own separate, distinct floodplain disconnected from the historic floodplain. According to the G&O study at ' U.S. 36 there is a low segment in the roadway northwest of the US Highway 36 bridge over South Boulder Creek where hydraulically disconnected flows depart from the main ' floodplain. This regulatory 100-year floodplain is shown on Figure 4. The perimeter ' berm around the gravel mine on the Flatiron Property had been constructed by the time the 1980 topographic mapping had been prepared. The G&O study assumed that the ' berm was a natural feature or a FEMA certified flood levee and that as such the berm would contain the flood waters between South Boulder Creek and the eastern edge of the ' berm. Upon completing its analyses, G&O re-mapped the Flatiron Property as being protected by the berm. This re-mapping, based upon the berm as a flood protection ' levee, removed the Flatiron mining operation from the floodplain. Also, the G&O study indicates that the north end of the Flatiron Property is partially inundated during a 100- ' year flood event by the westerly split flow of the South Boulder Creek floodplain upstream of U.S. Highway 36. However, the entire extent of flooding in this area is ' undefined. The G&O mapping ended abruptly east of the lone building existing on the Flatiron Property. The north end of the Flatiron Property also is crossed by the Viele Canal which traverses the northern property from east to west. The Viele Canal has a 100-year flood design discharge of approximately 960 cfs. 1 1 -g-